Colloidal silica sols are obtained industrially principally from waterglass, which is an inexpensive raw material. Waterglass is typically obtained by melting quartz sand together with soda or potash at temperatures around 1200° C. and then dissolving the quenched alkali metal silicate in water under pressure and elevated temperature.
In the customary process for preparing colloidal silica sols proceeding from waterglass, the latter is first treated with an acidic cation exchanger of the hydrogen type (cf., for example, “Colloidal Silica—Fundamentals and Applications”, Editors: H. E. Bergma, W. O. Roberts, CRC Press, 2006, ISBN: 0-8247-0967-5). The resulting silica having a pH of 2-4 is subsequently stabilized at a pH of 8-10 by alkalizing and heated to temperatures in the range of 80-100° C. to form particles. Suitable selection of the process parameters allows mean particle sizes in the range from 5 nm to about 200 nm to be established and the particle size distribution to be controlled.
As a result of the low purity of the starting materials, the colloidal silica sols prepared by this process, however, contain impurities, especially in the form of alkali metals and alkaline earth metals such as Na, K, Mg and Ca, and of transition metals such as Al, Fe, Cr, Ni, Cu and Zr in concentrations up to 1000 ppm, and the metals may be present in elemental form, in ionic form and/or as compounds, summarized hereinafter by the term “metal-based impurities (of metal x)”. Content figures for metal-based impurities, unless stipulated otherwise, are based here on the content of a metal/group of metals taking account of presence in elemental and ionic form and in the form of compounds in the possibly dissolved or dispersed SiO2-based solids, expressed in parts by weight (ppm) of the metallic element(s) in question. Particular applications, especially as an abrasive in chemomechanical polishing (CMP) processes which are used in the semiconductor and electronics industry to polish wafers of high-purity silicon, semiconductor materials and components, magnetic storage media and crystal substrates, in contrast, require high-purity colloidal silica sols which are essentially free of the impurities mentioned above.
In the chemomechanical polishing operation, the material is removed by a combination of a chemical etching operation of the polishing composition formulation and mechanical removal of the surface by the colloidal particles. Metal-based impurities of the colloidal silica sol used as an abrasive lead in this case to unwanted, disruptive effects which can severely impair the quality and functionality of the treated semiconductor material/product. For example, it is known that Na+ and K+ ions possess high mobility in semiconductor materials and can thus diffuse deep into the semiconductor material on contact of the polishing composition formulation with the semiconductor surface, as a result of which the electronic material properties change. On the other hand, Cu-based impurities have, for example, the pronounced property of enrichment at the treated semiconductor surface, as a result of which electrical short-circuit paths can be formed. Higher valency metal impurities, for example Al, Fe or Zr, can lead to increased scratch formation in the course of polishing. The reasons are yet to be clarified unambiguously, but it is possible, for example, that these impurities lead to the formation of larger particles (agglomeration or aggregation) which then correspondingly lead to scratch formation.
Processes for preparing a high-purity colloidal silica sol essentially free of metal-based impurities are already known. Ultrapure colloidal silica sols can be obtained, for example, via a sol-gel process by NH4OH-catalysed hydrolysis and condensation of an organic silane such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) in alcoholic solution (cf., for example, “Colloidal Silica—Fundamentals and Applications”, Editors: H. E. Bergma, W. O. Roberts, CRC Press, 2006, ISBN: 0-8247-0967-5). The colloidal silicas sols obtained in this way are notable, based on the very high purity of the starting materials, for very small amounts of metal-based impurities (ppb range). However, particular disadvantages are the high costs of the starting materials and residues of ammonia and organic solvent, which are undesirable for CMP applications.
The process explained in U.S. patent application Ser. No. 11/818,730, published as U.S. Patent Publication No. 2007/0254964 A1 for preparation of a high-purity colloidal silica sol (metal-based impurities in the range of 5-200 ppb, except Na at <1 ppm) is based on the use of a high-purity starting material, in this case fumed silicon dioxide. According to this, the fumed silicon dioxide is dissolved in aqueous alkali metal hydroxide solution, the alkali metal content is removed by means of an acidic cation exchanger and then particle formation is initiated by adjusting the temperature, the pH and the concentration of the silica solution. The fumed silicon dioxide used can be obtained, for example, by hydrolysing SiCl4 in an H2/O2 flame in an apparatus with a metal-free surface having an impurity content of <100 ppb (EP 1503957 A1). In addition to the complexity of the above-described sol preparation route, a disadvantage is again the use of comparatively costly starting materials.
There has thus been no lack of attempts in the past to develop processes for preparing colloidal silica sols in a purity sufficient for CMP applications (content of metal-based impurities in the low ppm range) based on the purification of technical alkali metal silicate solutions, especially waterglass, as a much less expensive raw material source. Typically, the aqueous alkali metal silicate solution, in a first step for removal of the alkali metal ions from the solution, is contacted with an acidic cation exchanger of the hydrogen type.
JP 2003-089786 A proposes, for example, a process for preparing a colloidal silica sol with a low content of metal-based impurities (alkali metal≤50 ppm; Cu<100 ppb; Zn<1000 ppb; Ca≤7 ppm, Mg≤10 ppm, Fe≤13 ppm) by purifying a 3-10% by weight aqueous alkali metal silicate solution. In this process, the aqueous alkali metal silicate solution is first contacted with an acidic cation exchanger of the hydrogen type to remove the content of metal-based impurities of alkali metals, then metal-based impurities of polyvalent metals are removed from the resulting silica solution with a pH in the range of 2-6 by passing it over a resin with chelate-forming functional groups and then alkalizing it with an amine or quaternary ammonium hydroxide to a pH of >8 at a temperature of 95-100° C. to induce formation of colloidal particles with a final particle diameter in the range of 5-150 nm. Optionally, the sol formation may be preceded by addition of oxidizing agents and/or soluble chelating agents, the latter serving, in the course of the final concentration of the sol by means of ultrafiltration to SiO2 content 10-60% by weight, to remove metal-based impurities in the form of water-soluble chelate complexes. The purity with regard to metal-based impurities of individual metals achievable by means of this process, especially of higher valency metals such as Zr and Al for example, is, however, considered to be inadequate for CMP applications.
The process described in EP 0537375 A1 for preparing aqueous colloidal silica sols of high purity proceeds from an aqueous alkali metal silicate solution in a concentration of 1-6% by weight as SiO2, which contains a content of metal-based impurities of 300-10 000 ppm. This is likewise passed in a first step over an acidic cation exchanger of the hydrogen type to remove the content of metal-based impurities of alkali metals, with an optional downstream anion exchange with a basic anion exchanger of the hydroxyl type. The resulting active silica solution (pH: 2-4) is adjusted to a pH of 0 to 2 by adding a strong acid and kept at a constant temperature in the range of 0-100° C. for a period of 0.1-120 h before it is contacted first with an acidic cation exchanger of the hydrogen type and then with a basic anion exchanger of the hydroxyl type to remove metal-based impurities and counterions introduced with the acid. To initiate particle growth, the resulting purified silica is subsequently introduced into a high-purity aqueous alkali metal silicate or alkali metal hydroxide solution heated to 60-150° C. over a period of 1-20 h until a molar SiO2/M2O ratio (M: alkali metal) in the range of 30-300 has been established. After ageing at a given temperature for a further 0.1-10 h, the stable sol formed with mean particle sizes in the range of 10-30 nm is concentrated to 30-50% by weight of SiO2 by means of a microporous membrane and then contacted in a final purification step first with an acidic cation exchanger of the hydrogen type, then with a basic anion exchanger of the hydroxyl type and subsequently with a further acidic cation exchanger of the hydrogen type. After final stabilization by adding ammonia, an aqueous silica sol (pH: 8-10.5) with a total content of metal-based impurities of polyvalent metals<300 ppm is obtained in this way. The content of metal-based impurities of alkali metals is, however, much too high for CMP applications at >800 ppm based on the SiO2 content of the sol.
JP 2006-036612 A describes a method for preparing high-purity aqueous silica solutions from aqueous alkali metal silicate solutions with a starting content of metal-based impurities of polyvalent metals in the range of approx. 100-40 000 ppm based on the addition of a water-soluble nitrogen- or phosphorus-containing chelating agent which forms anionic metal complexes with the metal-based impurities. These are removed from the solution by contact with a basic anion exchanger of the hydroxyl type—after an intermediate process step for removal of the alkali metal cations by ion exchange with an acidic cation exchanger of the hydrogen type. While the process enables a distinct reduction in the content of metal-based impurities of Zn by a factor of >18, the purifying effect with regard to the metal-based impurities of further elements (Cu, Mn, Ni and Fe) is much lower, and so a further purification before a step of colloid particle growth would be indispensible for CMP applications.
WO 2010/037702 A1 and WO 2010/037705 A1 describe processes for preparing high-purity solid SiO2 from silicate solutions by precipitation reaction. In this case, aqueous alkali metal silicate solution is added dropwise to an initial charge of a strong acid used in excess, and the pH should be kept within the range from 0 to less than 2 over the entire process. The shock-like transfer into the acidic medium leads to rapid gelation at the droplet shell, such that particles of good filterability with dimensions in the d50 range of 0.1-10 mm are precipitated, which can have different characteristic shapes depending on the solution viscosity and drop rate. The very low pH ensures that ideally no free negatively charged SiO groups, which can lead to the binding of troublesome metal ions, are present at the surface of the silica. The metal ions which are thus present in dissociated form can therefore be removed effectively from the precipitated silica by washing the filter cake. The silicon dioxide obtained after drying is notable for a comparatively high purity with regard to a broad spectrum of metal-based impurities with a content of Fe, Al, Ti≤5 ppm, of Ca, Ni, Zn≤1 ppm and of alkali metals≤10 ppm, and serves as starting material for the production of solar silicon. In view of the macroscopic particle sizes, this precipitated silica, however, is not immediately suitable for CMP applications.
In view of the deficits of the processes proposed to date, there is still a need for an effective and inexpensive process with a simple process regime for preparing aqueous colloidal silica sols in a purity which meets the requirements of use as an abrasive in chemomechanical polishing (CMP) processes in the semiconductor and electronics industries.